In continuous or long laser pulse emitting fiber laser amplifier systems, seed and pump radiation are typically supplied and guided away monolithically via passive transport fibers of the fiber amplifier unit.
In short pulse and ultrashort pulse fiber laser amplifier systems, the use of passive transport fibers is possible only to a limited extent, if at all, because the high intensities present in the fiber core (e.g., in passive transport fibers) can lead to non-linear effects. These effects can lead to self-phase modulation, which can destroy the pulse quality. With a chirped pulse amplification (CPA) laser system, such phase-modified pulses can no longer, or only incompletely, be compressed.
Alternatively, U.S. Pat. No. 9,014,220 B discloses a free-beam coupling. However, this coupling requires positioning optical components in relation to each other, where the positioning requires high accuracy and stability. Especially with thermal loads such as those due to fluctuations of the outside temperature during transport and waste heat created by the laser process itself (for example, by absorption, quantum defect, and photo-darkening) there can be strains or spatial changes within the fiber laser configuration. The latter can lead to deteriorations and instabilities when coupling in or out, whereby the coupling in is usually more critical for the performance of the fiber laser amplifier system.